1. Field of the Invention
The present invention relates to a method of producing a silicon wafer and an epitaxial wafer, and in particular to a method of producing a silicon wafer, an epitaxial wafer and a solid state image sensor, as well as a silicon wafer producing device, which enable easily forming a gettering site in a relatively short period of time and effectively suppressing occurrence of internal stress-induced dislocation, by irradiating a silicon wafer with predetermined types of laser beams.
2. Description of the Related Art
In recent years, mobile phones and digital video cameras include a high-performance solid state image sensor using a semiconductor mounted thereon, and hence the performances such as number of pixels and the like thereof have been remarkably improved. Such a consumer solid state image sensor as described above is required to be capable of capturing moving images with a relatively high pixel count and have a miniaturized size. In order to capture moving images in such a manner, it is necessary to combine the solid state image sensor with a high-speed computing element and a memory device, whereby a CMOS image sensor facilitating System-on-chip (SoC) is used and the downsizing of the CMOS image sensor is going on.
A dark leakage current of a photodiode is becoming a problem because it is a factor of deteriorating imaging properties of the solid state image sensor. The main cause of the dark leakage current is heavy metal contamination during processing steps. In order to suppress the heavy metal contamination, it has been generally attempted to form a gettering site for heavy metals at an inside of a semiconductor wafer or at a back surface of the wafer.
As means for forming a gettering site at the inside of a semiconductor wafer, there has been proposed a method (IG method) of subjecting the semiconductor wafer to a heat treatment to form an oxygen precipitate portion within the wafer, as disclosed in “Science of Silicon” edited by a UCS semiconductor substrate technique research institute, Realize Corporation, Jun. 28, 1996, p 585-590. In this method, however, a long-time heat treatment is required for forming the given oxygen precipitation portion, thereby possibly increasing producing cost and causing heavy metal contamination during the heat treatment.
As means for forming the gettering site at the back surface of a semiconductor wafer, there has been proposed a method (PBS method) of forming a polycrystalline silicon film on the back surface of the semiconductor wafer and using the back surface as a gettering site, as disclosed, for example, in “Science of Silicon”, edited by a UCS semiconductor substrate technique research institute, Realize Corporation, Jun. 28, 1996, p 585-590. However, when the semiconductor wafer has a large diameter of 300 mm or the like, in particular, it is difficult to form the gettering site on the back surface thereof because such a large-diameter wafer is usually a wafer to be polished on both surfaces thereof.
Further, recently, as a method of forming a gettering site at the inside of a double-side-polished semiconductor wafer such as a large diameter wafer having a diameter of 300 mm or more without necessitating a long-time heat treatment, there has been developed a method, as disclosed for example in Japanese Patent Application Laid-open No. 2008-108792, of irradiating the inside of a silicon semiconductor wafer with a low-output laser beam from a surface of the semiconductor wafer, thereby causing a multiphoton absorption process only at a predetermined depth position of the wafer to form a process-affected layer thereat, and utilizing the process-affected layer as a gettering site.